The security of information has been inevitably highly demanded in a network society. In communication technologies, various cryptographies have been developed against the threat of eavesdropping from ancient times. In recent years, there has been known that the cipher communication with very high security can be conducted by using the quantum-mechanical technique for the key distribution in the cipher communication. At present, the quantum cryptography is actively researched.
A system that has been currently most advanced in the research of the quantum key distribution and can be realized at the earliest time is a system that transmits a light that is made as weak as the number of photons can be counted one by one. The number of photons within one signal pulse is set to one or less on an average. Eavesdropping can be found out by this setting. Signal superposition is made by polarization modulation or phase modulation. The polarization modulation may be effective to a free space whereas the phase modulation may be effective to transmission using an optical fiber as a medium.
In order to make the eavesdropping difficult in the quantum key distribution, two kinds of modulated signal bases are prepared, for example, in protocol that is called “BB84”, and those bases are then selected at random according to the respective signals (see Non-patent Document 6, “N.Gisin, G. Ribordy, W. Tittel, and H. Zbinden, Reviews of Modern Physics 74, 145 to 195 (2002)”). In the polarization modulation, there are used two kinds of bases one of which allocates two linear polarizations to signals of “0” and “1”, and the other of which allocates two circular polarizations to signals of “0” and “1”. In the case of the phase modulation, there are used two kinds of bases one of which allocates phases 0 and π to signals of “0” and “1”, and the other of which allocates phases π/2 and 3π/2 to signals of “0” and “1”.
A reference light is required to detect the phase at a receiving side, and the reference light is transmitted to the receiving side together with a signal light. Then, the signal light and the reference light interfere with each other within a receiver to detect the phase. Because two kinds of bases are used in transmission of the signal, a function for selecting any one of the bases is required at the receiving side. One method for achieving the function is that the phase of the reference light is modulated to 0 and π/2 within the receiver.
A general single mode optical fiber is manufactured circularly symmetrically, but has a slight birefringence because of nonuniformity or bending. Therefore, even if, for example, a light of linear polarization is transmitted from a transmitting side, the transmitted light becomes generally an elliptic polarization. A normal phase modulator using the electrooptic effect has a polarization dependency because of an intensive birefringence of the electrooptic crystal. When the transmitted light becomes elliptic polarization and is modulated in phase, not only pure phase modulation but also polarization modulation is induced at the same time. In order to solve the above problem, a plug & play system has been devised (see Non-patent Document 1, “A. Muller, t. Herzog, B. Huttner, W. Tittel, H. Zbinden, and N. Gisin, Applied Physics Letters. 70, 793 to 795 (1997)”; and Non-patent Document 2, “H. Zbinden, H. Bechmann-Paquinucci, N. Gisin, and G. Ribordy, Appllied Physics B 67, 743 to 748 (1998)”).
The above system is designed in such a manner that a light source is disposed at not a transmitter side but a receiver side, and a light on which signal is superimposed is reciprocated between the receiver and the transmitter. A Faraday mirror is disposed at the transmitter so that when a light emitted from the receiver is reflected at the transmitter and returned to the receiver, the polarizations are always orthogonal to each other in the reciprocating lights. When a light emitted from the receiver is a linear polarization, even if the light is transmitted through any transmission path, the light that has been again returned to the receiver becomes the linear polarization that is orthogonal to the original light. Therefore, the normal phase modulator can be used at the receiver. The light is generally an elliptic polarization at the transmitter even if the light at the receiver is set to be a linear polarization. However, any polarization at the transmitter induces no problem if the phase modulator is disposed in proximity to the Faraday mirror and the lights are modulated in reciprocation since the two polarization components are evenly modulated in reciprocation. However, this method suffers from such a problem that a backward scattered light in the transmission path is mixed into the detector because the light source is disposed at the receiver. This problem causes a transmittable distance to be limited in the quantum key distribution using a faint light.
In order to solve the above problem, there have been recently proposed one-way transmission systems in which a light source is disposed at the transmitter to eliminate the influence of backward scattering (see Non-patent Document 3, “K. Inoue, E. Waks, and Y. Yamamoto, Physical Review Letters 80, 37902 (2002)”; Non-patent Document 4, “K. Inoue, E. Waks, and Y. Yamamoto, Physical Review A 68, 22317 (2003) “; and Non-patent Document 5, “Y. Nambu, T. Hatanaka, and K. Nakamura,. Japanese Journal of Applied Physics 43, L1109 to L1110 (2004)”). In all of those proposed methods, the receiver is made up of only a simple interferometer, and no phase modulator is disposed. However, a method for selecting the bases is required at the receiver.
In Non-patent Document 3, two or more delay lines are prepared at the transmitter, and one signal is made up of three or more pulses so that adjacent pulses are allowed to interfere with each other by means of a delay line that is disposed at the receiver. The three or more pulses composing one signal is transformed to four or more pulses at the receiver. Where a photon is detected in four or more pulses is probabilistic and cannot be known until the photon is detected. In this system, the bases at the receiving side are determined according to the position of the received pulse.
In Non-patent Document 4, a temporal coherence of the light is assumed, and the respective pulses are modulated by differential phase shift keying. The receiver determines 0 and π by a simple asymmetric Mach Zehnder interferometer. In this system, the bases are not selected at the receiver, and the detection of eavesdropping is based only on the fact that the number of photons in one pulse is less than one. This fact assures no case in which all of signals are eavesdropped on, and if any portion of the signals is eavesdropped on, the consecutive slots of the portion eavesdropped on are uncertain for an eavesdropper because the signal is differential. If the eavesdropper resends a signal, including uncertain slots, in order to conceal the eavesdropping, the receiver can detect the fact of eavesdropping.
In Non-patent Document 5, two Mach Zehnder interferometers are disposed at the transmitter, and one Mach Zehnder interferometer is disposed at the receiver. With this structure, the bases at the receiver are automatically determined from the slot at which a photon has been detected of three successive pulse slots.